1. Technical Field
The present invention relates to a fixing device and an image forming apparatus incorporating the same, and more particularly, to a fixing device that processes a toner image with heat and pressure on a recording medium, and an electrophotographic image forming apparatus, such as a photocopier, facsimile machine, printer, plotter, or multifunctional machine incorporating several of these features, which incorporates such a fixing device.
2. Background Art
In electrophotographic image forming apparatuses, such as photocopiers, facsimile machines, printers, plotters, or multifunctional machines incorporating several of those imaging functions, an image is formed by attracting toner particles to a photoconductive surface for subsequent transfer to a recording medium such as a sheet of paper. After transfer, the imaging process may be followed by a fixing process using a fixing device, which permanently fixes the toner image in place on the recording medium by melting and setting the toner with heat and pressure.
FIG. 1 is an end-on, axial view of a roller-based fixing device 310 employed in electrophotographic image formation.
As shown in FIG. 1, the fixing device 310 includes a pair of opposed rotary fixing members, one being a fuser roller 312 internally heated with a heat source 311 for fusing toner, and the other being a pressure roller 313 pressed against the heated roller 312, which together form a heated area of contact called a fixing nip N.
During operation, a rotary drive motor drives the fuser roller 312 to rotate in a given rotational direction (clockwise in the drawing), which in turn causes the pressure roller 313 to rotate in an opposite rotational direction (counterclockwise in the drawing). Heating of the fuser roller 312 is computer-controlled to maintain the roller surface at a temperature equal to or higher than the softening temperature of toner.
As the two rollers 312 and 313 rotate together, a recording sheet S bearing a toner image thereupon enters the fixing nip N, at which heat and pressure exerted from the opposed rollers 312 and 313 causes the toner to melt and fuse on the incoming sheet S. Then, as the recording sheet S exits the fixing nip N, the molten toner gradually cools and solidifies to fix in place on the recording sheet S.
FIG. 2 is an end-on, axial view of a belt-based fixing device 410 employed in electrophotographic image formation.
As shown in FIG. 2, the fixing device 410 includes a fuser belt 415 looped for rotation around a fuser roller 412 and a motor-driven heater roller 414 internally heated with a heat source 411, as well as a pressure roller 413 pressed against the fuser roller 412 via the fuser belt 415, which together form a fixing nip N therebetween.
During operation, a rotary drive motor drives the heater roller 414 to rotate in a given rotational direction (clockwise in the drawing), causing the belt 415 and the fuser roller 412 to rotate in the same rotational direction, which in turn causes the pressure roller 413 to rotate in an opposite rotational direction (counterclockwise in the drawing). Heating of the heater roller 414 is computer-controlled to maintain the belt 415 at a temperature equal to or higher than the softening temperature of toner.
As the fuser belt 415 and the rollers 411 through 413 rotate together, a recording sheet S bearing a toner image thereupon enters the fixing nip N, at which heat and pressure exerted from the opposed rollers 412 and 413 causes the toner to melt and fuse on the incoming sheet S. Then, as the recording sheet S exits the fixing nip N, the molten toner gradually cools and solidifies to fix in place on the recording sheet S.
Modern electrophotographic printers find application in high-quality printing processes with emergence of new imaging technologies. For example, the widespread use of digital cameras has led to an increased demand for printers that can reproduce digital image data on paper or other types of recording media as in the manner of analog, film-based photography. Also, with the development of print-on-demand (POD) technology which allows for small-lot, wide-variety printing services, there is a growing trend to replace a traditional offset printing press with an electophotographic printer, as the former involves costly production of printing plates and is typically less productive and less efficient than the latter.
A problem encountered when applying electrophotographic printing process to photographic image formation is insufficient gloss of an image printed and fixed on a recording medium. In general, a toner image processed through a roller-based or belt-based fixing device exhibits a gloss or reflectivity of about 60% at best. This level is significantly low when compared to that of a film-based photographic print which typically falls within a range from about 80% to 90%. The relatively low gloss of an electrophotographic print may be attributed primarily to minute damage to the printed surface caused where the recording medium, sticking to the fuser member due to adhesion of molten toner immediately after fixing, is forcibly detached from the fuser member.
Various techniques have been proposed to provide printing with high-gloss, photo-like imaging quality, several of which are directed to development of a more sophisticated fixing process.
For example, one known technique proposes a fixing device including an endless rotary fixing belt entrained around multiple rollers, including a heater roller and a stripper roller, parallel to each other, as well as a pressure roller pressed against the heater roller via the belt to form a fixing nip therebetween. Inside the loop of the fixing belt is a stationary, contact-cooling device, such as a heat sink, disposed in contact with the belt between the heater roller and the stripper roller to cool the belt downstream from the fixing nip.
Another known technique proposes a fixing device including an endless rotary fixing belt entrained around multiple rollers, including an upstream, inlet roller and a downstream, outlet roller, parallel to each other, with a pressure member pressed against the inlet roller via the belt to form a fixing nip therebetween. The downstream roller inside the loop of the fixing belt is configured as a hollow cooling roller accommodating a heat transfer fluid in its hollow interior, which absorbs heat from the belt and from the toner image downstream from the fixing nip.
According to this method, the fixing device is provided with an image gloss adjustment capability; a positioning mechanism for adjustably positioning the downstream roller to adjust a position at which a recording medium separates from the fixing belt, which in turn allows for adjusting an amount of heat removed from the toner image with the cooling roller. The fixing device thus adjusts an amount of gloss imparted to the resulting print by controlling the cooling efficiency of the cooling roller through the roller positioning mechanism.
Although generally successful for their intended purposes, the approaches depicted above have several drawbacks.
For example, employing a stationary, contact-cooling device for cooling the rotary belt results in sliding, frictional contact between the belt and the belt cooler, which move relative to each other where the former rotates while the latter does not during operation. Continuous friction between the belt and the belt cooler causes abrasion of their sliding surfaces, leading to possible damage to the belt as well as generation of dust particles from the worn surface of the belt cooler, which eventually migrate and cover the adjoining surfaces of the fuser roller and the stripper roller.
Presence of such particulate matter between the fixing roller and the belt can translate into a reduced area of contact, and therefore a reduced friction or traction between the roller and belt surfaces, which hinders proper rotation of the belt and causes various media conveyance failures, such as paper jams and multi-feeding. Further abrasion to the belt cooler can reduce an area of thermal contact between the belt and the belt cooler, leading to a reduced cooling efficiency which adversely affects glossing performance of the fixing device.
Also, employing a rotatable cooling roller, instead of a stationary cooling device, results in a reduced cooling efficiency due to a relatively small area of contact between the belt and the belt cooler, particularly where the cooling roller is positioned with a relatively low contact pressure against the belt. Increasing the contact pressure between the cooling roller and the belt to obtain a larger cooling efficiency is not practical, since it would in turn cause various concomitant failures, such as imaging defects, paper jams, and paper curls, in the fixing device.